Welding unit converter
The conversions that come up when a drawing, a WPS or a machine plate arrives in the units you do not work in.
The ones that catch people
cfh and l/min. Cubic feet per hour is the American unit for gas flow. Twenty cfh looks like a large number next to "10", but it is about 9.4 l/min — a perfectly normal setting. Read it as litres and you will run the arc unshielded.
ipm and m/min. Inches per minute for wire feed. 300 ipm is 7.6 m/min. Here the American number is the big one, and setting 300 on a metric machine is not possible, which at least fails safely.
kJ/in and kJ/mm. Heat input in AWS documents is per inch. A limit of 50 kJ/in is about 2 kJ/mm — a factor of 25 apart, and the same word for both.
| Inches | mm | Where you meet it |
|---|---|---|
| 1/16″ | 1.6 | Tungsten, thin filler |
| 3/32″ | 2.4 | Tungsten, electrodes |
| 1/8″ | 3.2 | The most common electrode |
| 5/32″ | 4.0 | Electrodes, tungsten |
| 3/16″ | 4.8 | Tungsten, thick filler |
| 1/4″ | 6.4 | Plate thickness |
Which system a document uses
If it cites EN ISO or ISO standards, it is metric. If it cites AWS, ASME or API, expect imperial units even when the drawing dimensions are metric — mixed documents are common on international projects, and the units are worth checking line by line rather than assuming.
Conversion factors
| Quantity | Metric | Imperial | Factor |
|---|---|---|---|
| Length, thickness | mm | inch (″) | ÷ 25.4 |
| Feed speed | m/min | ipm | × 39.37 |
| Gas flow | l/min | cfh | × 2.119 |
| Heat input | kJ/mm | kJ/in | × 25.4 |
| Pressure | bar | psi | × 14.50 |
| Mass | kg | lb | × 2.205 |
| Temperature | °C | °F | × 9/5 + 32 |
How to use the converter
- Pick the quantity in “What to convert” — length, wire feed speed, gas flow, heat input, pressure, mass or temperature.
- Set the direction: metric to imperial or the other way.
- Type the value exactly as it appears in the document, without rounding. The result appears at once, with the reverse ratio underneath — useful when several numbers have to be converted in your head.
Round only the final result, and only to the precision of the original. A gas flow of 35 cfh is 16.5 l/min; writing “16.52” into a WPS claims a precision the flowmeter does not have.
Examples from a WPS and a catalogue
| In the document | Working | Metric |
|---|---|---|
| 3/16″ plate | 0.1875 × 25.4 | 4.76 mm |
| .035″ wire | 0.035 × 25.4 | 0.89 mm — in practice 0.9 wire |
| .045″ wire | 0.045 × 25.4 | 1.14 mm — sold as the 1.2 equivalent |
| 300 ipm wire feed | 300 ÷ 39.37 | 7.6 m/min |
| 35 cfh gas | 35 ÷ 2.119 | 16.5 l/min |
| 2900 psi cylinder | 2900 ÷ 14.50 | 200 bar |
| 38 kJ/in heat input | 38 ÷ 25.4 | 1.5 kJ/mm |
| 300 °F preheat | (300 − 32) × 5/9 | 149 °C |
| 33 lb spool | 33 ÷ 2.205 | 15 kg |
Inch and millimetre wire sizes do not match exactly: .045″ is 1.14 mm, not 1.2. Choose the contact tip and drive rolls for the wire actually on the feeder — see MIG/MAG torch consumables.
ksi, psi and MPa: strength in American standards
AWS and ASTM give strength in ksi — thousands of pounds per square inch. The factor is a physical constant: 1 ksi = 1000 psi = 6.895 MPa. Examples:
- E7018 — the “70” is a minimum tensile strength of the weld metal of 70 ksi: 70 × 6.895 ≈ 483 MPa. In the metric edition of the same AWS specification the electrode is called E4918 — the class is rounded to 490 MPa.
- ASTM A36 — yield strength 36 ksi: 36 × 6.895 ≈ 248 MPa. Close to S235, but not the same steel — see steel grade equivalents.
- The other way: S355 is 355 ÷ 6.895 ≈ 51.5 ksi.
Mind what is being counted: in the AWS electrode classification the first digits are tensile strength, in EN ISO 2560-A they are yield strength. So E7018 and E 42 4 B are not “70 = 42” — they are two different quantities.
Conversion traps
- Heat input the wrong way round. 1 kJ/mm = 25.4 kJ/in, not the reverse; getting the direction wrong puts the result out by a factor of 645.
- A temperature difference treated as a temperature. “Hold within ±50 °F” is ±28 °C: a difference is only multiplied by 5/9, without subtracting 32.
- Sheet thickness in gauge. A gauge number is not a unit of length: 16 ga steel is 1.52 mm, 16 ga aluminium 1.29 mm. Always know which gauge table applies.
- Gauge and absolute pressure. US data sheets write psig or psia; a regulator gauge shows gauge pressure, just like bar on a European gauge.
Frequently asked questions
What is cfh?
Cubic feet per hour, the American measure of gas flow. 20 cfh is about 9.4 l/min. Read it as litres per minute and you weld with half the gas — then wonder about the porosity.
kJ/mm or kJ/inch?
European documents work in kJ/mm, American ones in kJ/inch. The factor is 25.4 — large enough to notice, easy to apply the wrong way round. 1 kJ/mm is 25.4 kJ/inch.
Why is there no hardness conversion here?
Because that is not a conversion but a tabulated correspondence that depends on the material. It has its own calculator, based on ISO 18265.
How many MPa is 1 ksi?
1 ksi = 1000 psi = 6.895 MPa. So an E7018 electrode has a tensile strength of at least 70 × 6.895 ≈ 483 MPa, and ASTM A36 steel a yield strength of 36 ksi ≈ 248 MPa. Going the other way, divide by 6.895: 355 MPa is about 51.5 ksi.
What metric wire matches .030, .035 and .045 inch?
Converted, they are 0.76, 0.89 and 1.14 mm; the trade treats them as the equivalents of 0.8, 0.9 and 1.2 mm wire. The sizes are not identical, so match the contact tip and rolls to the wire actually on the spool.
How do I convert a temperature difference from °F to °C?
Multiply by 5/9 only, without subtracting 32. A tolerance of ±50 °F is ±28 °C, and a rise of 100 °F is 55.6 °C. The formula (°F − 32) × 5/9 applies only to an actual temperature, for example 300 °F = 149 °C.
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